1. Field of the Invention
The present invention relates to a semiconductor memory device. More particularly, the present invention relates to a semiconductor memory device including a single transistor having functions of random access memory (RAM) and read-only memory (ROM), and methods for operating and manufacturing the same.
2. Description of the Related Art
While RAMs allow fast data access, they are volatile, meaning that they lose their contents when the power is turned off. In contrast, ROMs, which are computer memories on which data has been prerecorded, retain their contents even when the power is turned off. ROM has a data access rate that varies depending on the type of memory, but is much slower than that of RAM. For example, flash memories, which are a variation of electrically erasable programmable read-only memory (EEPROM) that can be written and read by an electrical signal, are similar to ROM in that they retain their contents even when the power is turned off. At the same time, the flash memories have characteristics similar to RAM in that they can be erased and reprogrammed. Flash memories are faster than EEPROM, but they are not as fast as RAM. Also, since flash memories can write data by injecting electric charges through tunneling, their write voltages are high and their write time is long.
To overcome the above problems, nonvolatile RAMs (NVRAMs) have been developed. To get a NVRAM, a dielectric film of a capacitor in 1T-1C dynamic RAM having an array of unit cells, each having a single transistor and a single capacitor, is replaced by a capacitor with a ferroelectric film such as PZT (PbZrxTi1-xO3), and an electrode is replaced by a heat resistant metal such as platinum (Pt). However, the NVRAM has a problem in that the fabrication process is complicated because a new process recipe is used to pattern a ferroelectric material and a heat resistant metal that are hard to etch. Another problem of NVRAM is that the direction of voltage applied must be changed for data recording and writing, and it is not commercially viable due to high applied voltage. Furthermore, it is more difficult to effectively utilize space than if a flash memory is used since the NVRAM includes capacitors together with transistors.